1. Field of the Invention
The present invention relates to a power hand tool and more particularly, to the assembly structure of an impact drill.
2. Description of the Related Art
A conventional power hand tool comprises an electrical motor and a spindle rotatable by the electrical motor to provide an output torque. Subject to different applications, conventional power hand tools include electrical screwdrivers, electrical wrenches and electrical hand drills. Further, to overcome the resistance encountered during operation, conventional power hand tools have a function to output an impulse force intermittently.
For example, when an electrical drill encounters a resistance, the spindle will be forced to move intermittently in an axial direction, outputting an impulse force and overcoming the resistance. This axial striking action is called hammer.
Further, when an electrical wrench encounters a resistance during its operated to fasten a hexagonal bolt to a workpiece or unfasten it from the workpiece, the electrical wrench forced to move intermittently in a radial direction (the direction of rotation), outputting an impulse force and overcoming the resistance. This radial striking action is called impact.
However, when operating an electrical hand drill to drill a hole on a workpiece, the spiral cutting edge of the drill bit encounters a resistance in an axial direction as well as in a radial direction. In this case, the hammer function can simply overcome the resistance in the axial direction but cannot overcome the resistance in the radial direction. Under certain situations, the impact function helps drilling performance of the electrical hand drill. FIGS. 5 and 6 show a drill chuck 100 for use with an electrical wrench 200, for example, De Walt model DW0521. The drill chuck 100 has a hexagonal rod 110. The electrical wrench 200 has a coupling hole for receiving the hexagonal rod 110 so that the electrical wrench 200 is usable as an electrical drill. By means of the impact effect, the electrical wrench 200 works efficiently during a drilling operation. However, because this design simply uses the hexagonal rod 110 to achieve transfer of a rotary driving force between the electrical wrench 200 and the drill chuck 100, the drill chuck 100 tends to vibration during a drilling operation, affecting hole size control.
FIG. 7 shows another structure of electrical wrench 300 that has a rectangular spindle 310 for the mounting of a drill chuck 400. After connection of the drill chuck 400 to the electrical wrench 300, the assembly is used as an impact drill. However, this structure is a detachable design, having a relatively greater range of tolerance to facilitate mounting and dismounting of the drill chuck 400. Therefore, when the drill chuck 400 is sued with the electrical wrench 300 to drill a hole, the drill chuck 400 tends to vibration, not suitable for high precision drilling application.